Fluids produced from an underground formation, is usually a three-phase mixture of water, oil and gas, in which at least some of the water is emulsified in the oil. This mixture of fluids is usually separated into its phases downstream of the wellhead, in order to be deliverable to pipelines for further distribution. A possible method for performing such a separation is to use a three-stage process, with two gravity separator tanks followed by a last separator tank including an electrostatic coalescer. The gravity separators usually include a flow straightener at the inlet. The purpose of the flow straightener is two-fold; to equalize the flow regime across the section of the tank, and to affect a mechanical coalescing of water droplets prior to the separation by gravity, in order to increase the efficiency of the separation process.
The problem with this multistage process is that separation is most effective at the first stage, where there is a large percentage of water to be separated out. At the last stage, the percentage of water is relatively low, which means that an electrostatic coalescer must be introduced in this gravity separator tank.
Still, the efficiency is low, which means that the fluids have to remain in the separator tank for a long time in order to allow the phases to separate out. Thus, the tank has to be large, e.g. about four meters in diameter and 20 meters long. It is inconvenient to use a tank of this size at the production site, especially offshore where the available space is limited.
Prior Art
U.S. Pat. No. 4,469,582 describes an electrically enhanced inclined plate separator in a downstream compartment connected to an electrical system to generate electric fields within the passages of the separator to coalesce and separate a polar is liquid (water) from a non-polar liquid (oil). The separator generally consists of an array of parallel flat, or corrugated, plates (electrodes) through which the processed mixture of liquids passes. Each plate is made of two sections. The first section is made of en electrically conductive material. The downstream, second section is made of a non-conductive material.
U.S. Pat. No. 4,919,777 describes a treater for electrostatic/mechanical separation of brine from oil during longitudinal flow through a horizontally elongate tank, wherein coalescer elements are provided for enhancement of the de-emulsification process. The emulsion is directed through electrical fields where the brine droplets take on an electrical charge, then moved through an electrically grounded coalescing element comprising a multiplicity of longitudinally-extending downwardly-inclined open-ended tubes arranged in a bundle-fashion.
A drawback of the separators disclosed in U.S. Pat. Nos. 4,469,582 and 4,919,777 is that they both use bare electrodes. A coalescer with bare electrodes in contact with the fluids would not withstand the condition of being flooded with water. The saline water is conductive and will short-circuit the electrodes, rendering the electrical system inoperative. With bare electrodes, water content of 10% is normally regarded as the limit. Also, such bare electrodes would be subjected to corrosion due to the emulsion. Thus, these can only be used late in a chain of settling tanks, where the water content has been brought down below the 10% limit.